Apple's GarageBand is a great entry-level music application for OSX. It comes with a decent selection of default samples to play with, but if you want to branch out and find some more MacBand.com is a good place to start.

MacBand is an online directory of songs and loops made by artists using Apple Computer's GarageBand software. Use MacBand to share your music with the world...(or)...Browse the directory to download songs and loops made by other MacBand artists.